Gas turbine power systems have become popular systems for meeting modern society's power needs. Not only do gas turbines provide thrust for most large aircraft, but they also have been adapted for use in generation of electricity in stationary power plants.
Gas turbines operate on the Brayton cycle and have a working fluid, typically air, which remains gaseous throughout the cycle. While the Brayton cycle can theoretically be closed so that the working fluid recirculates, the vast majority of operational gas turbine power plants operate as an open Brayton cycle. In the open Brayton cycle commonly found in commercial stationary power plants, air is drawn into a compressor where its pressure and temperature increase. The temperature of the air is then further increased by combusting a fuel, most often natural gas, in the air to produce a working fluid including air (minus the oxygen which reacts with the fuel) and the products of combustion of the oxygen and the fuel (typically carbon dioxide and steam). This high temperature high pressure working fluid is fed into a turbine where the working fluid is expanded and its temperature and pressure decreased. The turbine drives the compressor and typically additionally drives a generator for the generation of electric power. The working fluid is exhausted from the turbine in a simple open Brayton cycle.
Most operational stationary gas turbine power systems include a simple open Brayton cycle only as one part of a combined cycle. Specifically, because the working fluid still has a relatively high temperature when exiting the turbine, this heat can be used, such as to generate steam in a heat recovery steam generator before finally being exhausted. The steam heated within the heat recovery steam generator can then be utilized to drive a steam turbine, such as that found in any typical closed Rankine cycle steam power plant. When operated as a combined cycle, the open Brayton cycle gas turbine and closed Rankine steam turbine combine to most efficiently extract power from the fuel combusted within the gas turbine (in some systems over 50% thermal efficiency).
While significant advances in compressor and turbine designs have greatly increased the efficiency with which the gas turbine operates and have increased the temperature at which the gas turbine can operate, the gas turbine has certain drawbacks. One drawback of the open Brayton cycle gas turbine is that the exhaust includes oxides of nitrogen (NOx). NOx is a pollutant which can only be emitted in compliance with strict environmental regulations within the United States. Also, open Brayton cycle gas turbines emit carbon dioxide (CO2) into the atmosphere. While emission of CO2 is not currently regulated by the United States government, mounting scientific evidence has connected the emission of CO2 with global warming and other negative atmospheric effects. Numerous proposals are being evaluated for regulation of the emission of CO2. Accordingly, a need exists for a way to eliminate the emission of NOx, CO2 and other pollutants from gas turbines.
Techniques do exist for reduction of the emission of NOx and the elimination of CO2 from open Brayton cycle gas turbine exhaust. The exhaust can be scrubbed of a significant portion of the NOx by various different processes applied to the exhaust to either convert (i.e. using ammonia) or separate the NOx from the exhaust. Such “scrubbers” not only decrease the efficiency of the operation of the gas turbine, they are costly and also fail to remove all of the NOx from the exhaust.
Large quantities of CO2 are produced within the open Brayton cycle gas turbine as one of the major products of combustion of the natural gas in air. This CO2 is exhausted in gas form mixed with the large amount of nitrogen in the air which passes through the gas turbine. If removal of the CO2 is desired, the CO2 must first be separated from the nitrogen and other gaseous constituents of the working fluid (i.e. by chemical and/or physical absorption of the CO2, or endothermic stripping processes for separating CO2 from the exhaust gases). The CO2 can then be used in industrial processes or can be eliminated, such as by pressurization and sequestration in underground or deep sea locations. While such CO2 sequestration is a known technique, significant energy is utilized in separating the CO2 from the nitrogen, and hence the efficiency of the open Brayton cycle gas turbine is significantly decreased when CO2 separation is required. Accordingly, a need exists for more efficient separation of the CO2 from other portions of the working fluid so that the efficiency of the gas turbine is not radically diminished.
Closed Brayton cycle gas turbines have been developed for certain specific applications, such as for gas turbines which operate in nuclear power plants. In the closed Brayton cycle a working fluid is provided (typically Helium in nuclear power plants) which remains separate from the heat source and which is recirculated from the turbine exhaust back to the compressor without ever being exhausted. The compressor typically needs modification when the gas being compressed is changed. Because the working fluid is not exhausted, it is not a source of atmospheric pollution. The heat source which heats the working fluid can be nuclear, solar, geothermal or some other form of renewable non-polluting heat source so that atmospheric emissions are avoided.
However, if combustion of a hydrocarbon fuel with air is utilized to heat the working fluid between the compressor and the turbine, the closed Brayton cycle gas turbine will still have an exhaust which includes CO2 and NOx. While renewable non-polluting heat sources such as nuclear, solar and geothermal are effective, they suffer from drawbacks which have limited their ability to be fully competitive with hydrocarbon fuel combustion powered gas turbine systems. Other closed Brayton cycle or partially closed Brayton cycle gas turbine power systems have been proposed which utilize a mixture of CO2 and oxygen as the combustion medium. For instance, see U.S. Pat. No. 5,724,805 to Golomb. While such CO2 closed Brayton cycle gas turbine systems do keep nitrogen out of the combustor and so do not produce NOx, the high density of CO2 makes it ill suited for use within a compressor which has been designed for compression of air. Accordingly, a need exists for a Brayton cycle gas turbine which heats the working fluid by combustion of a hydrocarbon fuel and which avoids emission of pollutants into the environment.
Another known technique for modifying prior art open Brayton cycle gas turbines is to inject steam into the combustor upstream of the turbine. When steam is injected into the combustor, the power output and the efficiency of the open Brayton cycle can be enhanced. Various different prior art steam injection open Brayton cycles are disclosed by Wilson and Korakianitis in The Design of High-Efficiency Turbo Machinery and Gas Turbines, Second Edition, 1998, Prentice-Hall, Inc. For instance, Wilson and Korakianitis cite one study by the General Electric Corporation that their LM5000 gas turbine, when fitted with steam injection and intercooling will experience a power increase from 34 MW to 110 MW and an efficiency improvement from 37% to 55%, compared to a simple Brayton cycle gas turbine power system with no associated Rankine cycle. Such steam injection open Brayton cycles typically do not operate as part of a combined cycle, but rather utilize the heat recovery steam generator to turn feed water into steam for injection upstream of the turbine. Hence, by steam injection, high efficiencies and high power outputs are provided without requiring a separate steam turbine and condenser as required for a combined cycle.
Steam injection open Brayton cycles also suffer from numerous drawbacks. Such cycles require feed water purification to keep the machinery in good working order. Water purification costs hence impede the desirability of prior art steam injection open Brayton cycles. Also, prior art steam injection open Brayton cycles still produce NOx, carbon dioxide and other pollutants which are emitted into the atmosphere as with the non-steam injection open Brayton cycle gas turbine power systems described above.
The prevalence of open Brayton cycle gas turbines and particularly combined cycle gas turbine power systems throughout the world which are emitting large amounts of NOx and CO2 into the environment makes desirable the provision of a method and apparatus for retrofitting open Brayton cycle gas turbines in a manner which does not interfere with the existing equipment but which eliminates emission of nitrogen oxides, CO2 and other pollutants into the atmosphere, so that capital costs associated with such retrofits can be minimized. Such retrofits would additionally benefit from the use of a working fluid which matches the performance characteristics of working fluids in known prior art open Brayton cycle systems so that optimum performance of the system components can be maintained.